1. Field of the Invention
This invention relates to methods and apparatus for processing modem signals in networks and more particularly to methods and apparatus for processing modem signals in networks having voice compression.
2. Description of Related Art
Modems are commonly used to transmit digital data signals d(n) over digital or analog voice networks. A first modem connected to a point of entry of a voice network generates an analog voice signal s(t) containing the digital data signal d(n) by modulating the digital data signal d(n) using known modulation techniques such as V.22bis and V.32bis modulation techniques. A second modem connected to a point of exit of the voice network converts or demodulates the transmitted analog voice signal s′(t) into a digital data signal d′(n) representing the original digital data signal d(n) subject to data loss. In digital voice networks, one source of data loss may be packet loss, i.e., one or more digital packets representing the voice signal s(t) may be lost during transmission from the first modem to the second modem across the digital voice network.
Another source of data loss may be due to digital voice compression of the voice signal s(t) as it is converted to a digital voice signal s(n) and transmitted over the digital voice network. A normal voice signal s(t), when digitized s(n), uses a bandwidth of 64 kbps for transmission across a digital network. This bandwidth enables the transmission of modem signals as well as voice signals with very low distortion. As noted, however, may digital voice networks compress voice signals s(t) prior to transmission as digital packets containing a digital signal s(n) representing the voice signal s(t). FIG. 1 is a block diagram of a prior art transmission system for transmitting voice signals s(t).
As shown in FIG. 1, the system includes a digital voice encoder 2, digital voice network 30, and digital voice decoder 4. The digital voice encoder 2 converts a received voice (or modem signal) s(t) into a plurality of digital packets representing a digital version s(n) of the voice signal. The digital voice network 30 transports the plurality of digital packets to the voice decoder 4. As noted above, the digital voice network 30 may be a lossy system, i.e., one or more of the plurality of digital packets may not be delivered or delivered out of order. Voice decoder 4 receives the digital packets representing a digital voice signal s′(n) from the digital voice network 30 and generates a voice/modem signal s′(t) from the received digital packets.
As shown in FIG. 1, the voice encoder may include a voice compression unit 16. The voice compression unit attempts to remove redundant speech information from the received speech signal s(t). As noted above, digital speech signals s(n) are commonly 64 bps signals prior to compression. After compression, the digital speech signals s(n) may be 8 kbps or less. As noted, the compression unit 16 attempts to remove redundant speech information and consequently may distort or corrupt modem signals. Some compression units may distort or corrupt the modem signals so severely that the modulated signal can not be demodulated without losses.
To counter for or reduce such error losses, the data rate of a digital signal d(n) modulated into a voice signal s(t) to be transmitted by a network having voice compression may be reduced. For example, the maximum data rate of a digital signal d(n) modulated into a voice signal s(t) that is processed by a compression unit 16 using the well known, low distortion ADPCM (G.721) 32 kpbs compression algorithm is 4800 bps. Thus, even though the compressed digital voice signal s(n) is a 32 kpbs signal, only a 4800 bps digital signal d(n) can be transmitted across a digital voice transmission system 5 employing such a compression unit. This is a highly inefficient use of bandwidth considering the voice compression unit 16 is added to reduce bandwidth consumption. Thus, a need exists for a digital voice transmission system 5 that more efficiently transmits modulated digital signals as well as compressed voice signals.